Background: The natural cosmetics market is expanding, and milk, valued for its biological properties and low toxicity, is gaining popularity as a cosmetic ingredient due to its moisturizing, anti-inflammatory, and anti-aging effects. Mare’s milk, distinct from cow’s milk, offers superior microbiological quality and potential as a luxury product, though it remains underutilized in Poland’s cosmetics industry. This study examined the hygienic quality of mare’s milk and soaps derived from it. Methods: The study was conducted on a stud farm with twenty-five mares and two stallions of the Sztumski breed, under strict hygiene and feeding standards. Physicochemical and microbiological analyses of mare’s milk and the resulting soaps included assessments of nutrient content, microbiological testing, and challenge tests conducted in accordance with ISO 11930 to evaluate antimicrobial properties and product safety. Results: The milk showed high microbiological quality, low fat (0.64–0.96%) and protein (1.70%) content, and a high lactose level (6.61%). Most soap samples were free of microbial growth, demonstrating their hygienic status and effective production decontamination. Although some preservatives showed limited efficacy against specific microorganisms, three soap samples remained resistant to contamination throughout the 28-day challenge test. Conclusions: Overall, mare’s milk soaps proved safe and stable. Improvement of their formulation could further enhance their stability and competitiveness in the natural cosmetics market.